Meiosis

As discussed when talking about the cell cycle, one of the major functions of cell division is reproduction. As humans, we, like many eukaryotes, use the process of meiosis for this process. Meiosis is a type of cell division that produces gametes - the specialized sex cells, sperm and egg.

To aid in the comprehension of this process, it is essential to know two terms:

Somatic cells, like those produced at the end of mitosis (including the germ cells that will be producing gametes) are diploid. Gametes are haploid.

Interphase

Like for the mitotic cell cycle, the meiotic cell cycle begins with Interphase, and it works exactly the same way.

G1

The first step in Interphase is G1. Just think of "G" for "growth". In this phase, the cell grows as it prepares to divide. It also produces more organelles, and synthesizes mRNA and proteins that will be required for the next step.

S

If one cell is going to become two cells, it needs to have enough DNA to support both cells. S Phase is where new DNA is synthesized. Through a highly regulated and conserved process, DNA Replication occurs and copies the DNA.

G2

The final step of Interphase is G2. Once again, the cell will grow and produce proteins as it finishes preparing to divide.

Meiosis I

After Interphase is M Phase, where the cell actually divides. In meiosis, there are actually two divisions, named "meiosis I" and "meiosis II". While the steps of meiosis broadly follow the same structure as mitosis, there are some key differences.

Prophase I

Just like in mitosis, the nucleus begins to breakdown. Chromatin will condense into chromosomes (the more tightly-wound "X" that you probably think of as existing in the nucleus).  Centrosomes, as they move to opposite ends of the cell, will begin the production of spindle fibers.

A major event here that was absent in mitosis is crossing over. Homologous chromosomes enter synapsis as they tightly pair up. These pairs are called "tetrads" as there are four sister chromatids. Double-strand DNA breaks occur and are repaired in such a way that the homologous chromosomes essentially "swap" pieces. Chiasmata form at the location of synapsis, holding the homologous chromosomes together.

As the homologous chromosomes swap pieces, crossing over very often results in the creation of new combinations of genes. When this occurs, it is known as recombination.

What are homologous chromosomes?

Homologous chromosomes are chromosomes that have the same genes, in the same order/locations, but that might have different versions of these genes. We have two homologous chromosomes for each chromosome - one is a maternal chromosome and one is a paternal chromosome.

Want to learn about the substages of Prophase I?

There's a lot going on in Prophase I and it takes up most of the time of meiosis. It is split into 5 distinct parts based on what's occurring with the chromosomes:

Prometaphase I

In prometaphase, the nucleus finishes breaking down. The kinetochore, a protein complex, forms at the centromere of each chromosome. Spindle fibers attach to these kinetochores and will be essential in moving the chromosomes.

Metaphase I

The motor proteins associated with the spindle fibers move the chromosomes such that they line up along the metaphase plate at the center of the cell.

Due to the chiasmata holding the homologous pairs together, they line up as pairs, rather than single file.

Anaphase I

During this phase, the chiasmata are broken. Homologous chromosomes are pulled apart and moved to opposite ends of the cell. 

It is random which homologous chromosome ends up on which side -during Metaphase I - a phenomenon known as Independent Assortment. As the homologous chromosomes segregate due to Anaphase I, that randomness impacted which chromosomes will end up in each daughter cell. This increases variation by increasing the number of possible combinations in each daughter cell.

Telophase I

During telophase I, a new nucleus forms around each set of chromosomes. The chromosomes decondense back into chromatin. The spindle fibers dissemble. 

Cytokinesis I

This step of the cell cycle actually usually begins in late anaphase and continues throughout telophase. This is the step where the cytoplasm divides. 

As the sister chromatids did not separate, each daughter cell that is produced is haploid.

This process began with a diploid germ cell. At the end of meiosis I, we have two genetically different haploid cells.

Meiosis II

There is no second Interphase in this process. The cells will immediately move from meiosis I into meiosis II. Each daughter cell made in meiosis I will do the following steps, although only one is shown in the images for simplicity.

Meiosis II works essentially the same way as mitosis, except we are beginning with haploid cells where the sister chromatids are not identical anymore.

Prophase II

The nucleus begins to breakdown. Chromatin will condense into chromosomes.  Centrosomes, as they move to opposite ends of the cell, will begin the production of spindle fibers.

Prometaphase II

In prometaphase, the nucleus finishes breaking down. The kinetochore, a protein complex, forms at the centromere of each chromosome. Spindle fibers attach to these kinetochores and will be essential in moving the chromosomes.

Metaphase II

The motor proteins associated with the spindle fibers move the chromosomes such that they line up single-file along the metaphase plate at the center of the cell.

Anaphase II

As sister chromatids did not separate in Anaphase I, we still have replicated chromosomes up until this point. During this phase, the sister chromatids are pulled apart and moved to opposite ends of the cell. 

Telophase II

A new nucleus forms around each set of chromatids. The chromosomes decondense back into chromatin. The spindle fibers dissemble. 

Cytokinesis II

The cytoplasm once again divides. This results in the formation of four haploid cells that are genetically different from each other.